Since the development of continuous pulp digesters for pulp production, there has been a need to accurately measure pulp consistency of the pulp slurry in the discharge blowline at the discharge of the digester. The pulp consistency in the pulp slurry at the discharge of the digester together with pulp flow rate are the key variables for retention time control of the pulping process which affects the uniformity of the pulp produced.
The retention time of the cellulose fibers in the digester has a significant effect on the degree of delignification of the wood chips from which the cellulose fibers are released to produce a pulp slurry in which a desired amount of lignin is dissolved. Therefore, assuming all other conditions of the digestion process are constant, the shorter the retention time in the digester, the larger the content of lignin remaining within the cellulose fibers in the pulp slurry. Conversely, the longer the retention time in the digester, the smaller the lignin content remaining within the cellulose fibers in the pulp slurry.
There is presently no method to directly track the retention time of a given amount of cellulose (wood chips) in the digester. Therefore, indirect methods are used to determine the retention time in the continuous digester via measurement of certain characteristics of the pulp slurry in the discharge blowline of the digester. For example, systems have been devised to attempt to estimate pulp consistency in the blowline by measuring the power consumption of an outlet device which pumps the pulp slurry into the blowline. Systems have also been devised for measuring the differential pressure along the discharge blowline of the digester to determine pulp consistency. However, these types of measurement systems are influenced by the Kappa number, which is a measure of the residual lignin remaining with the cellulose fibers, and by the production rate. Granberg et al., Kappa Number Control of Kamyr Digesters, EUCEPA Symposium on "Control Systems in Pulp and Paper Industry," May 11-14, 1982, Stockholm, Sweden. Although such systems have met with some success, their overall acceptance has been limited because of their dependence on the Kappa number.
Attempts have been made to determine pulp consistency in the blowline by the combined use of a refractometer and a gamma gauge. In these systems, the refractometer and gamma gauge are disposed adjacent to the blowline for making measurements of certain characteristics of the pulp slurry in the blowline. The refractometer attempts to measure the density of the liquid phase of the pulp slurry, while the gamma gauge attempts to measure the total density of the pulp slurry in the blowline. From these measurements pulp consistency in the blowline was determined. U.S. Pat. Nos. 4,146,422 and 4,239,590; and Canadian Patent Nos. 1,002,361 and 1,035,620. This type system worked in some paper mills but other paper mills had problems due to refractometer failure. Granberg et al., Kappa Number Control of Kamyr Digesters, EUCEPA Symposium on "Control Systems in Pulp and Paper Industry," May 11-14, 1982, Stockholm, Sweden.
Furthermore, there are other substantive problems associated with the use of a refractometer to make measurements of the density of the liquid phase of the pulp slurry. One is that deposits can accumulate on the optical window of the refractometer in contact with the pulp slurry thereby preventing any accurate measurement by this device. Another is that the refractive index of any liquid is function of the liquid density and liquid composition; and since the latter depends on the organic and inorganic content of the liquid phase of the pulp slurry, measurements made by a refractometer can be inaccurate because organics usually have higher refractive indices than inorganics, but their densities have the opposite relationship. So, the measurements made by the refractometer are dependent on the composition of liquid phase of the pulp slurry immediately adjacent to the optical window of the refractometer.
The present invention overcomes these problems and provides a system and method to more accurately and reliably determine pulp consistency.